1. Field of the Invention
The present invention relates to an energization control apparatus for a glow plug used, for example, for pre-heating a diesel engine.
2. Description of the Related Art
Conventionally, a glow plug which generates heat upon supply of electric current is used to assist startup of an engine of an automobile or the like, or to stably operate the engine. A glow plug is composed of a heating resistor (a heating coil, a ceramic heater, or the like); a control coil and a center rod, which serve as a path for supplying electric power to the heating resistor; a metallic shell; etc.
Heat generation of such a glow plug is controlled by an energization control apparatus. For example, when an engine key is turned on, a large amount of electric power is supplied to the glow plug within a short period of time so as to raise the temperature of the glow plug to a temperature sufficient for starting an engine. In general, such energization for rapidly raising the temperature of the glow plug is called a “pre-glow” or a “pre-glow step.” Furthermore, after the glow plug has reached the above-mentioned temperature and the engine has started, for a predetermined period of time, electric power is supplied to the glow plug in order to maintain the temperature of the glow plug at a predetermined temperature. In general, such energization for maintaining the temperature of the glow plug is called “after glow” or an “after glow step.”
In recent years, in order to prevent malfunctions caused by a drop in temperature within a combustion chamber following the after glow, such as generation of black smoke at a time when the engine is rapidly brought into a high speed or high output state when the combustion chamber is cold, a technique of re-energizing the glow plug has been proposed to cause the glow plug to again generate heat (perform an intermediate temperature raising) during a period after completion of the after glow while the engine is operating (see, for example, Patent Document 1). This technique is described in detail. First, a first intermediate temperature raising means calculates a duty ratio on the basis of a target resistance corresponding to a target temperature of a glow plug, and controls, by means of PWM (pulse width modulation) control, the supply of electric current to the glow plug in accordance with the duty ratio. Subsequently, a second intermediate temperature raising means calculates the duty ratio on the basis of a voltage applied from a battery to the glow plug, and then controls, by means of PWM control, the energization of the glow plug in accordance with the duty ratio. By providing the first intermediate temperature raising means and the second intermediate temperature raising means, the temperature of the glow plug can be raised rapidly at the initial stage of the intermediate temperature raising operation. Also, in a state in which the temperature of the glow plug approaches a target temperature, the temperature of the glow plug can be stably maintained at the target temperature.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2005-240707
3. Problems to be Solved by the Invention
Since the duty ratio is calculated on the basis of the voltage applied from a battery to a glow plug, maintaining the glow plug at a constant temperature is difficult in the case where the glow plug is thermally influenced from the outside, such as the case where the heating resistor of the glow plug is cooled because of a disturbance caused by a change in engine speed, load (throttle opening), water temperature, etc. In order to maintain the temperature of the glow plug constant, it is necessary to obtain information regarding the engine speed, load, etc., from, for example, an ECU, and control the effective voltage applied to the glow plug on the basis of the information thus obtained. However, if a to-be-applied effective voltage is calculated in accordance with changes in various parameters, such as engine speed, load, and water temperature, the processing load on the ECU may increase. In order to reduce such a processing load, the energization control apparatus may be configured to use a previously prepared map which can univocally determine the to-be-applied effective voltage from the above-mentioned various parameters and a target temperature, and which can perform energization control on the basis of the map. However, such a map must be prepared by changing the above-mentioned various parameters in various ways such that the to-be-applied effective voltage can be determined univocally in all situations. Therefore, it causes an increase in troublesome preparation for creating the map, and the period of time required to complete the map becomes longer.
In over to overcome such drawbacks, energization control for a glow plug may be performed through control based on the resistance of the glow plug alone. However, in such a case, the following problem may arise. The value measured as the resistance of the glow plug is not the resistance of the heating resistor, but the sum of the resistance of the heating resistor, the resistances of a control coil and a center rod, the resistance of a power supply harness connected to the glow plug, and the resistance of a metallic shell. Furthermore, as the target resistance, the resistance of the glow plug is set at the time when the temperature of the glow plug becomes saturated at the target temperature (that is, the control coil, the center rod, etc., also become saturated). Accordingly, in the case where resistance control is performed such that the resistance of the glow plug coincides with the target resistance, the following problem occurs. In an initial stage of the intermediate temperature raising operation (in a stage in which the temperatures of the control coil, the center rod, etc., have not yet become saturated), the ratio of the resistance of the heating resistor to the resistance of the glow plug is relatively large. Therefore, electric power, which is normally used to raise the temperatures of the control coil, etc., is used to cause the heating resistor to generate heat, whereby the temperature of the heating resistor is raised rapidly. Therefore, overshoot (excessive temperature rising) of the glow plug (the heating resistor) may arise. Furthermore, since the temperature of the heating resistor is gradually transmitted to the control coil, the center rod, etc., after that time, the ratio of the resistance of the heating resistor to the resistance of the glow plug decreases. Therefore, if energization control is performed with the target resistance maintained constant, the temperature of the heating resistor becomes lower than the target temperature. That is, the temperature of the glow plug, which has once increased excessively, decreases at this time. Therefore, the temperature of the glow plug may vary greatly around the target temperature, and a relatively long period of time may be required for allowing the temperature of the glow plug to stabilize the target temperature.